1. Field of the Invention
The present invention relates to a hybrid vehicle which comprises an engine and a motor as power sources, and in which the power of at least one of the engine and the motor is transmitted to driving wheels for driving the hybrid vehicle.
2. Description of the Related Art
In general, in hybrid vehicles of the above-mentioned type, two drive zones, i.e., a drive zone in which an engine is operated (hereinafter this zone is referred to as an “engine drive zone”) and another drive zone in which a motor is operated (hereinafter this zone is referred to as a “motor drive zone”), are prepared in advance, and the present drive mode is appropriately determined in one of the drive zones so as to improve fuel economy.
To this end, conventionally, the drive mode is determined in one of the engine drive zone and motor drive zone based on an opening degree signal of an accelerator, or alternatively, the drive mode is appropriately switched according to a map in which the engine drive zone and motor drive zone are defined, respectively, based on parameters such as an opening degree signal of an accelerator, a vehicle speed, an engine revolution rate (or motor revolution rate), etc.
In the hybrid vehicle disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 1-153330, the opening degree of the throttle of an engine and power of a motor are controlled based on a vehicle speed and the opening degree of an accelerator.
However, in the above conventional hybrid vehicles, because the drive mode is determined with reference to the fixed engine drive zone or motor drive zone, the drive mode may be switched too frequently since the drive mode may be switched due to small movement of the accelerator operated by the driver, or due to changes in vehicle conditions and environment. Moreover, switching of the drive mode may be delayed even when an urgent acceleration is required, and in addition, the motor may be used less frequently so that fuel economy is degraded.
Furthermore, in the above conventional hybrid vehicles, because the engine is controlled according to the fixed engine drive zone and motor drive zone, a further problem is encountered in that the drive mode is not quickly switched to the engine drive mode upon request of an urgent acceleration when the vehicle runs in the motor drive mode, i.e., response of the vehicle is not sufficient.